Conventional winter tires (hereinafter, also simply referred to as tire(s)) provide grip on icy road surfaces via stud pins installed into a tread portion of the tires.
Typically, when a tire including a stud pin travels on icy road surfaces, the ice powder dug up by the stud pins may enter the space between the tread surface and the icy road surface. Such a layer of ice powder reduces the ability of the stud pins to break into the ice. As a result, the grip on icy road surfaces provided by the stud pins is decreased.
Whereas, a studdable tire is known that is capable of providing high steering stability performance by suppressing the coat of ice powder to the stud pin, the ice powder dug up while traveling on icy road surfaces (Japanese Patent No. 5571207).
Such a studdable tire includes intra-land grooves and communicating grooves communicating the intra-land grooves with the lug grooves. Each intra-land groove has both ends thereof terminating within a land portion and formed on each of a step-in side and a kick-out side of the land portion within a region enclosed by two circles of different radii having a center concentrical to a center of a stud hole on a land surface of a tread.
Additionally, a stud pin with improved performance on ice is known (International Patent Application Publication No. WO/2014/102936). This stud pin includes a shank portion that has an end surface, a flange portion provided on a base end of the shank portion, and an uneven portion disposed on the end surface of the shank portion. The uneven portion is constituted of either convex portions or concave portions with respect to a ground contact reference plane.
The studdable tire described above including the intra-land grooves and communicating grooves disposed around the stud hole on the land surface of the tread can reduce ice powder attached to stud pins, however the degree to which this is achieved is not always sufficient.
Additionally, the recess portion, which is concave with respect to the ground contact reference plane, radially extending centered at the center of the tip surface where a tip is disposed of the stud pin cannot reduce ice powder attached to the stud pin.